A prior art publication “Photonic Crystals” by R. Wehrspohn, ISBN3-527-40432-5, p. 238-246 discloses a sensor using a photonic crystal. The sensor uses a three-dimensional photonic crystal of bulk-type as a sensing element, and is configured to introduce a target gas through one of faces opposed with respect to the thickness of said sensing element, and to guide a light having a wavelength in match with an absorption wavelength of the target gas through the one face of the sensing element, thereby detecting the light emitted from the other face of the sensing element by use of a detector such as a photo-detector for calculating a gas density based upon the intensity of the detected light.
Generally, a group velocity Vg of the electromagnetic wave propagating within the photonic crystal is defined by Vg=(dβ/dω)−1, where β is the propagation constant, and ω is a frequency. Therefore, the group velocity Vg becomes lower as a ratio of variation in the frequency ω to the variation of propagation constant β becomes lower, and becomes zero when the relation between frequency ω and propagation constant β satisfies a standing wave condition (end condition of waveguide mode).
The sensor disclosed in the publication is designed to elongate an optical path length by setting the group velocity Vg propagating in the three-dimensional photonic crystal to be about 30% of the light velocity in the vacuum, and is therefore required to give a thickness (i.e., a dimension along the direction of incident light) of several centimeters to the three-dimensional photonic crystal. In this consequence, the three-dimensional photonic crystal is required to have a uniform cyclic structure of refractive index in the order of 100 nm that satisfies the condition of the frequency ω and the propagation constant β for obtaining the intended group velocity. However, when the cyclic structure of refractive index distorts, the group velocity goes out of the intended value at the distorted portion, which fails to make accurate density measurement. Therefore, the three-dimensional photonic crystal is required to be fabricated with an extremely precise manufacturing technology
Further, since the three dimensional photonic crystal has a relatively large thickness, the propagation mode becomes multi-modes including the slower group velocity mode and higher group velocity mode, which may lower the sensitivity than with the constant group velocity. Also, since the sensitivity may vary when the coupling efficiency to various modes with respect to the incident light, the three dimensional photonic crystal is required to be positioned accurately in relation to the light source.
Further, since the electric field intensity distribution in space for the light of lower group velocity Vg is considerably different from the Gaussian distribution in space of the normal light, a complicated coupling structure for conversion of the electric field intensity distribution is required in order to avoid a light coupling loss which would be otherwise seen at the light incident face of the three dimensional photonic crystal to lower the sensitivity.